Graphite and molybdenum disulfide (MoS2) are the predominant materials
used as solid lubricant. In the form of dry powder these materials are effective lubricant
additives due to their lamellar structure. The lamellas orient parallel
to the surface in the direction of motion.

Even between highly loaded stationary surfaces the lamellar
structure is able to prevent contact. In the direction of motion the
lamellas easily shear over each other resulting in a low friction. Large particles best perform on relative rough surfaces at low
speed, finer particle on relative smooth surface and higher speeds.

Solid lubricants are useful for conditions when conventional
lubricants are inadequate.

reciprocating motion. A typical application is a sliding
or reciprocating motion that requires lubrication to minimize wear
as for example in gear and chain lubrication. Liquid lubricants will
squeezed out while solid lubricants don't escape and prevent for
fretting corrosion and galling.

ceramics. Another application is for cases where
chemically active lubricant additives have not been found for a
particular surface, such as polymers and ceramics.

high temperature. Graphite and MoS2 exhibit high
temperature and oxidizing atmosphere environments, whereas liquid
lubricants typically will not survive. A typical application include
fasteners which are easily tightened and unscrewed after a long stay
at high temperatures.

extreme contact pressures. The lamellar structure orient
parallel to the sliding surface resulting in high bearing-load combined
with a low shear stress. Most applications in metal forming that
involve plastic deformation will utilize solid lubricants.

Graphite

Graphite is structurally composed of planes of polycyclic
carbon atoms that are hexagonal in orientation. The distance of carbon
atoms between planes is longer and therefore the bonding is weaker.

Graphite is best suited for lubrication in a regular atmosphere.
Water vapor is a necessary component for graphite lubrication. The
adsorption of water reduces the bonding energy between the hexagonal
planes of the graphite to a lower level than the adhesion energy
between a substrate and the graphite. Because water vapor is a
requirement for lubrication, graphite is not effective in vacuum. In
an oxidative atmosphere graphite is effective at high temperatures up
to 450ºC continuously and can withstand much higher temperature peaks.
The thermal conductivity of graphite is generally low ~1.3 W/mK at
40ºC.

Graphite is characterized by two main groups: natural and
synthetic. Synthetic graphite is a high temperature sintered product
and is
characterized by its high purity of carbon (99.5-99.9%). The primary
grade synthetic graphite can approach the good lubricity of quality
natural graphite.

Natural graphite is derived from mining. The quality of natural
graphite varies as a result of the ore quality and post mining
processing of the ore. The end product is graphite with a content of
carbon (high grade graphite 96-98% carbon), sulfur, SiO2
and Ash. The higher the carbon content and the degree of
graphitization (high crystalline) the better the lubricity and
resistance to oxidation.

For applications where only a minor lubricity is needed and a more
thermally insulating coating is required, then amorphous graphite
would be chosen (80% carbon).

Molybdenum Disulfide

MoS2 is a mined material found in the thin veins
within granite and highly refined in order to achieve a purity
suitable for lubricants. Just like graphite has MoS2
a hexagonal crystal structure with the intrinsic property of easy
shear. MoS2 lubrication performance often
exceeds that of graphite and is effective in vacuum as well whereas
graphite does not. The temperature limitation of MoS2
at 400ºC is restricted by oxidation. The particle size and film
thickness are important parameters that should be matched to the
surface roughness of the substrate. Large particles may result in
excessive wear by abrasion caused by impurities in the MoS2,
small particles may result in accelerated oxidation.

Boron
Nitride

Boron Nitride is a ceramic powder lubricant. The most interesting
lubricant feature is its high temperature resistance of 1200ºC service
temperature in an oxidizing atmosphere. Further Boron has a high
thermal conductivity. Boron is available in two chemical
structures, i.e. cubic and hexagonal where the last is the lubricating
version. The cubic structure is very hard and used as an abrasive and
cutting tool component.

PTFE

PTFE is widely used as an additive in lubricating oils and greases.
Due to the low surface energy of PTFE, stable unflocculated
dispersions of PTFE in oil or water can be produced. Contrary to the
other solid lubricants discussed, PTFE does not have a layered
structure. The macro molecules of PTFE slip easily along each other,
similar to lamellar structures. PTFE shows one of the smallest
coefficients of static and dynamic friction, down to 0.04. Operating
temperatures are limited to about 260ºC.

Application
methods

Spraying/dipping/brushing: Dispersion of solid lubricant as an
additive in oil, water or grease is most common used. For parts that
are inaccessible for lubrication after assembly a dry film lubricant
can be sprayed. After the solvent evaporates, the coating cures at
room temperature to form a solid lubricant. Pastes are grease like
lubricants containing a high percentage of solid lubricants used for
assembly and lubrication of highly loaded, slow moving parts. Black
pastes generally contain MoS2. For high
temperatures above 500°C pastes are composed on the basis of metal
powders to protect metal parts from oxidation necessary to facilitate
disassembly of threaded connections and other assemblies.

Free powders: Dry-powder
tumbling is an
effective application method. The bonding can be improved by priory
phosphating the substrate. Use of free powders has its limitations,
since adhesion of the solid particles to the substrate is usually
insufficient to provide any service life in continuous applications.
However, to improve running-in conditions or in metal forming
processes a short duration of the improved slide conditions may
suffice.

AF-coatings: Anti-friction coatings are "lubricating paints"
consisting of fine particles of lubricating pigments, such as
molydisulfide, PTFE or graphite, blended with a binder. After
application and proper curing, these lubricants bond to the metal
surface and form a dark gray solid film. Many dry film lubricants also
contain special rust inhibitors which offer exceptional corrosion
protection. Most long wearing films are of the bonded type but are
still restricted to applications where sliding distances are not too
long. AF-coatings are applied where fretting and galling is a problem
(such as splines, universal joints and keyed bearings), where
operating pressures exceed the load-bearing capacities of ordinary
oils and greases, where smooth running in is desired (piston,
camshaft), where clean operation is desired (AF-coatings will not
collect dirt and debris like greases and oils), where parts may be
stored for long periods of time.

Composites

Self lubricating composites: Solid lubricants as PTFE,
graphite, MoS2 and some other anti friction and
anti wear additives are often compounded in polymers and all kind of
sintered materials. MoS2 for example is
compounded in materials for sleeve bearings, elastomere O-rings,
carbon brushes etc. Solid lubricants are compounded in plastics to
form a "Self lubricating" or "Internally lubricated" thermoplastic
composite. PTFE particles for example compounded in the plastic form a
PTFE film over the mating surface resulting in a reduction of friction
and wear. MoS2 compounded in Nylon reduces wear,
friction and stick-slip. Furthermore it acts as a nucleating agent
effecting in a very fine crystalline structure. The primary use of
graphite lubricated thermoplastics is in applications operating in
aqueous environments.